Graft versus host disease (GVHD) is the major complication associated with allogeneic hematopoietic stem cell transplantation. Damage to the gastrointestinal (GI) tract from GVHD is a particularly serious event leading to significant morbidity and mortality. Proinflammatory cytokines play a critical role in the pathophysiology of intestinal GVHD, in part, by activating donor T cell populations which subsequently induce direct tissue damage. In preliminary studies, we have identified interleukin 23 (IL-23) as a pivotal cytokine positioned at the apex of this cytokine cascade that secondarily activates a unique CD4+ IL-23 receptor (IL-23R)-positive T cell population that expresses the ?2 integrin, CD11c. The overall goal of this proposal is to define the mechanistic pathways by which this CD4+ T cell subset induces pathological damage and determine the regulatory pathways that counter balance IL-23R-mediated inflammation within the GI tract. Our overall hypothesis is that the ?2 integrin, CD11c, defines a novel, pathogenic CD4+ IL-23R+ T cell population that induces colonic inflammation during GVHD, and that is regulated by the Stat3-dependent cytokines, interleukin-10 and interleukin-27. Studies in Specific Aim 1 will be conducted to confirm that CD11c co-expression defines a colitogenic CD4+ IL-23R+ T cell population, determine whether Stat3 dependent cytokines produced during GVHD augment coordinate expression of the IL-23R and CD11c, and examine whether the pathogenicity of ?2 integrin-expressing CD4+ T cells is attributable to augmented gut-homing molecule expression on this T cell subset. We will also determine whether this ?2 integrin-expressing CD4+ T cell population is able to induce endothelial damage and neovascularization in the colon microenvironment. Studies in Specific Aim 2 will define mechanisms by which CD4+ IL-23R+ T cells are regulated during GVHD. We will test the hypotheses that IL-10 and IL- 27, which are also Stat3 dependent cytokines, play critical roles in the regulation of IL-23R-mediated inflammation in the colon. Proposed experiments will identify and characterize the specific IL-10- producing donor T cell populations most critical for regulating CD4+ IL-23R+ T cells. We will also determine how IL-27 modulates IL-23R-mediated inflammation in the GI tract through effects on both conventional and regulatory T cell populations. These studies will take advantage of unique reagents and transgenic mouse models which will allow us to characterize and define the effects of each of these cytokines in the pathophysiology of gastrointestinal GVHD. The overall objective of this proposal is to develop new insights into the pathophysiology and regulation of GVHD within the GI tract that will foster the development of clinically relevant strategies to mitigate this complication in allogeneic hematopoietic stem cell transplant recipients.